Dithiol adipates



United States Patent DITHIOL ADIPATES direct and mesne assignments, toGulf Research &

Development Company, Pittsburgh, Pa., a corporation of Delaware NoDrawing. Application Aug. 7, 1953, Serial No. 373,059

4 Claims. (Cl. 260-455) This invention relates to esters, and moreparticularly it is concerned with new chemical compounds which areespecially suitable for use as synthetic lubricants, i. e., forproviding a lubricating film between metal surfaces moving in contactwith and relative to each other.

The new compounds of our invention are dithiol esters selected from thegroup consisting of di-Z-ethyl-hexyl dithiol adipate, di-iso-octyldithiol adipate and di-2-octyl dithiol adipate. In other words, theesters of our invention are di-esters of adipic acid and Z-ethyl-hexyl,isooctyl and 2-octyl (secondary octyl) mercaptans.

The compounds of this invention are clear viscous liquids. ly in excessof about 400 F. at mm. Hg, a high flash point, a 'low pour point and ahigh viscosity index, all of which properties make them particularlysuitable for use as synthetic lubricants. The presence of sulfur in themolecule renders our esters particularly advantageous for use in' thoselubricant applications where good load carrying characteristics arerequired. The esters are quite stable and can be distilled attemperatures well over 400 F. without decomposition.

The dithiol esters of our invention can be :prepared by any suitablemethod, for example, by direct esterification between the adipic acidand the selected "mercaptan. However, generally speaking, directesterification does not ordinarily give high yields of the desiredproduct, and it is therefore preferred to prepare the esters by reactionof the mercaptan with an adipyl halide. In the latter produre, it isdesirable to add stoichiometric amounts 'of pyridine to react with thehydrogen halide formed in the reaction. The pyridine-hydrohalideseparates as a crystalline material and can be removed from the reactionmix- All of them have high boiling points, general- 2,710,300 PatentedJune 7, 1955 "ice Example I.Iso-octyl alcohol was obtained from the 0x0process using heptenes as the charge stock. The heptenes, in turn, wereobtained as a copolymer of propylene and butylenes in the presence of aphosphoric acid catalyst, the fraction of the copolymer boiling between170 and 210"F. and having a heptene content of about '90 to 100 per centbeing used as the charge stock. The corresponding rnercaptan wasobtained from the iso-octyl alcohol in known manner by converting thealcohol to the bromide and reacting the bromide with an equimolar amountof thiourea followed by reaction with sodium hydroxide. It should beunderstood that, as used herein and -in the appended claims, the termiso-octyl refers to thebranched chain octyl radicals obtained intheiso-octyl alcohol made by the 0x0 process.

Iso-octyl mercaptan, as prepared above, was charged in an amount of 293grams (2 mols) to a 1-liter, S-necked flask equipped with stirrer,reflux condenser, pot thermometer and dropping funnel. Pyridine, in anamount of 200 .grams (2 /2 mols), was also added to the flask. Whilestirring vigorously and with cooling to maintain the temperature at 20C. or below, 191 grams (1 mol) of adipyl chloride (95 per cent purity)were added to the flask over a period of about two hours. During theaddition the reaction temperature varied from l5-20 C. Early in theaddition of the adipyl chloride, pyridine hydrochloride separated out asa white granular precipitate, and benzene was added tofacilitate-stirring during the latter part of the addition when aconsiderable amount 3 of the precipitate had accumulated. After all theadipyl chloride had been added, the reaction mixture was stirred at roomtemperature for about one hour, and then the temperature was raised to90-95 C. for two hours to Y complete the reaction. The reaction mixturewas filtered at room temperature to remove the pyridine hydrochloride.The benzene was removed from the filtrate by fractionation through acolumn packed with glass helices. Thereafter, the distillation wascontinued under greatly reduced pressure in a still in which the pot wasconnected directly to the still head. The product distilling at 470'475F. at 0.8 mm. was the desired product, di-iso-octyl dithiol adipate, andwas obtained in a yield of 83.5 per cent. It contained 15.70 per cent ofsulfur, as determined by analysis, as compared to a calculated amount of1592 per cent.

Typical properties of the esters of our invention are shown in Table I.

TABLE I Sulfur Analysis, Melting Specific R I Percent Boiling Point, F.Point, Gravity, ,5

. Found Theory Di-2-ethyl-hexyl dithiol adipate. 428 at 0.2 mm. -75 1.4927 15. 15. 92 Di-lso-octyl dithiol adlpate 4724at 0.8 mm. 1. 4863 15.70 15. 92 Di-2-octy1 dithiol adipate 482 at 10 mm 0. 9665 1. 4912 14.15. 92

ture by filtration. -The ester product is recovered by fractionaldistillation; the yields obtained are on the order of 8095 per cent.

The mercaptans employed in the preparation of our esters can be obtainedfrom the corresponding monohydric alcohols in any known manner, e. g.,(1) by converting the alcohol to the bromide and reacting the bromidewith an equimolar amount of thiourea followed by reaction with sodiumhydroxide, (2) by reacting an alkyl halide with sodium hydrosulfide inalcoholic solution, (3) by catalytically reacting hydrogen sulfide andthe alcohol in the vapor phase, etc.

The following example is illustrative of one method of preparing theesters of this invention.

As has been indicated previously, the esters of our invention areparticularly suited for lubricant applications. The requirements forsatisfactory lubricants vary widely, depending upon the specificmechanism, engine or apparatus to be lubricated and the conditions underwhich the lubricating function is exercise. Thus, in many instances, itis required that a lubricant possess a minimal change of viscosity withchange of temp ratur i. e., a resistance to excessive thinning at hightemperatures and excessive thickening at low temperatures as indicatedby a high viscosity index. Similarly, the viscosity at any giventemperature, the pour point, flash point, load carrying capacity, etc.of a lubricant may be critical in certain applications. Frequently, alubricant must possess such a com- 3 bination of properties as to enableits use over a wide variety of conditions.

As shown in Table II, the esters of our invention exhibit a combinationof properties which make them su- 4 Furthermore, it has also been foundthat the dithiol esters are significantly superior to the correspondingoxygen esters from the standpoint of reducing wear and increasing loadcarrying capacity. This is shown in Table perior lubricants for manyapplications. 5 IV. Where still greater improvement in extreme-pressureTABLE II Viscosity in Oentistokes P our Flash Compound fifggg P olnt, rm.

210 F. 130 F. 100 F. 65 F.

DI-Z-ethyl-hexyl dithiol adipate. a. 12 7. s9 12. 84 13, 111 132 -7s 450Di-iso-oetyl dithiol adipate 3. 5s 8. 71 13. as 11, 941 150 70 405Di-2-octyl dithiol adipate 3. 2s 8. 34 13.78 12,948 121 -70 The estersof our invention are highly superior for the lubrication of turbopropand turbojet aircraft engines. Such engines must function attemperatures as low as 65 F. so that the lubricant must be sufiicientlyfluid at that temperature to permit engine starting. At the same time,the lubricant must also perform satisfactorily once the engine reachesoperating temperatures. As a practical maximum, it has been indicatedthat a lubricant must not have a viscosity in excess of about 13,000centistokes at temperatures of -65 F.; nevertheless, the viscosity athigher temperatures, such as at 100 F. and 210 F., should preferably bein excess of about 11.0 and 3.0 centistokes, respectively. In additionto these requirements, the lubricant must also have a minimum flashpoint of 385 F. and a pour point on the order of 70 or -75 F. or below.As shown in Table II, our esters can meet these stringent requirements.

It will be understood by those skilled in the art that for certainlubricant applications, the esters of our invention may require theaddition of certain additive compounds in order to meet specialrequirements. For example, antioxidants such a2,6-di-tertiarybutyl-4-methyl phenol and phenothiazine, anti-foam agentssuch as liquid dimethyl silicone polymers, and extreme-pressure agentssuch as tricresyl phosphate can be added to the ester lubricants of ourinvention to improve one or more properties thereof.

It has been proposedin the prior art to use certain diesters ofmonohydric alchols and aliphatic dicarboxylic acids as bases forsynthetic lubricants". Surprisingly, it has been found that the dithiolesters of this invention have superior viscometric properties to thecorresponding oxygen esters. Thus, the viscosities of the dithiol estersare at least 40 per cent higher and the viscosity indices are to percent higher than the corresponding oxygen esters.

gen esters and areton the order of below to F. The superior Viscometricproperties of the dithiol esters to the corresponding alcohol esters areclearly shown in Table III.

Notwithstanding this, the pour points of the di- F thiol esters aresimilar to those of the corresponding oxyproperties is required, extremepressure agents can be added to the esters, as indicated earlier in thisspecification.

TABLE IV Friction tests on thiol ester type synthetic lubricants FalexTests Wear Modified I Steel-Steel Herschel Test Material -Pound SeizureCoeflicient oi Gage Load. Steel-Steel, Static Friction 3-HT. TestSeconds at Lb.

Period Load Steel-Steel Wear: N0. of Teeth, Total Di-iso-octyladipate.-. 34 45 at 1,250..- 0. 24 Di-iso-octyl dithiol 25 4,000 l 0. 20

adipate.

References Cited inthe file of this patent UNITED STATES PATENTS a Catlin et a1. Mar. 7, 1950 OTHER REFERENCES Marvel et al.: J. A. C. 8.,vol. 73, pp. 1100-1102 (1951). Murphy et al.: Ind. & Eng. Chem, vol. 45,pp. 1766 1774 (August 1953).

1. AS NEW COMPOUNDS, DITHIOL ESTERS SELECTED FROM THE GROUP CONSISTINGOF DI-2-ETHYL-HEXYL DITHIOL ADIPATE, DIISO-OCTYL DITHIOL ADIPATE ANDDI-2-OCTYL DITHIOL ADIPATE.